It is well-known that when an IGBT (Insulated Gate Bipolar Transistor) is in the conduction state, excess minorities are injected from its emitter region into its base region and thereby the concentration of non-equilibrium carriers can be significantly higher than that of equilibrium carriers, resulting a strong conductivity modulation in base region and therefore the voltage drop under conduction is significantly decreased. FIG. 1 shows the structure of a cell of a typical n-IGBT and its equivalent circuit. It can be seen from this figure that the IGBT is a bipolar junction transistor (BJT) driven by an n-Metal-Insulation-Semiconductor Field Effect Transistor (n-MISFET), wherein the emitter E and collector C of the bipolar transistor constitute the emitter and collector of the IGBT, respectively, and the base current of bipolar transistor is the drain current of n-MISFET which is controlled by the gate electrode G. Since bipolar carriers participate in electric conduction in IGBT, the switching speed is slowed down inevitably. Especially, there is a significant current tail during the turn-off period. The conventional methods to improve the switching speed includes: employing the anode short-circuit structure; reducing the lifetime of non-equilibrium carriers in base region and diminishing the emitter injection efficiency.
FIG. 2 shows a schematic diagram of the structure of a common IGBT with anode short-circuit in Ref. [1]. When the IGBT is turned on and used for large current applications, the electrons that flow from the channel region into the base region of BJT directly flow out of the n-region at the bottom, and a voltage drop will be formed when the electrons flow through the n-region. That is to say, there is a voltage drop across p-region and n-substrate. When the voltage drop is greater than about 0.7 V, a large amount of holes are injected from p-region into n−-region, and then conductivity modulation effect occurs in the base layer. When the IGBT is used for small current applications, the voltage drop across the p-region and n−-substrate is smaller, which makes the injection efficiency of the IGBT emitter junction lower, and a much small amount of holes are injected into the base region. Therefore the injection efficiency of the emitter of the common anode-short IGBT will decrease as the emitter current IE decreases. When the emitter current IE decreases to a certain value, there will be no hole injected from the emitter region into the base region, thus diminishing the phenomenon of current tail. The methods such as using technology of controlling lifetime of non-equilibrium carriers as well as reducing the emitter injection efficiency for increasing the switching speed are to some extent by decreasing the current produced by non-equilibrium carriers.
However, the above methods are at the expense of the conductivity modulation effect when the device is in the conduction state, which makes the increase of the voltage drop in the on-state. In addition, the methods mentioned above can not entirely eliminate the current tail when the device is turning off, but rather to achieve a tradeoff between switching speed and on-state voltage drop.